THE Broad Walk is one of London’s leafiest avenues, stretching between two corners of Hyde Park. On a cold but sunny spring day, it is a welcome escape from Park Lane’s noisy traffic, luxury hotels and car showrooms.

I’m taking a walk with Ian Rodger, who manages trees for the city’s Royal Parks. “These were planted in about 1860,” says Rodger, gazing up at the giant London planes on either side of the path. Their canopies spread out high above him and fuzzy fruits fall occasionally from clusters in the branches.

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The Victorians planted thousands of these trees because they thrive in any soil and their flaking bark and glossy leaves made them impervious to the soot and smoke of the industrial revolution. But as we have learned to our cost, their roots can spread widely, disrupting roads, pavements and buildings. “What the Victorians didn’t know was how bloody big they get,” says Rodger.

Nor could they have appreciated the true value of the trees, beyond their hardiness and handsomeness. In fact, that is something we are only just coming to fully appreciate. Recently, a band of “treeconomists” have begun to put a fair price tag on trees, accounting for the services they provide, from keeping our buildings cool to preventing skin cancer. The results are sometimes startlingly large – and can help people like Rodger plead the case for our cities’ trees.

When trees hit the headlines, it is often in the context of deforestation. But in most developed countries, forests have actually been …